There is a longstanding need in the manufacture and service of prime movers, particularly of diesel engines exceeding 250 horsepower in power output, and of heavy duty vehicles such as highway trucks and coaches, for dynamometers, or test stands, which need far less cooling water than has been required prior to the present invention.
Water brakes used as absorption dynamometers or motion retarders utilize fluid friction or momentum exchange of water, or other liquid, to dissipate mechanical power delivered from a connected power source or power transmission shaft, such as the crankshaft of an engine to be tested, an axle shaft to be slowed, or the like, through use of a rotor, operating in a bath or spray of water or other liquid within a stator. Following the first law of thermodynamics regarding conservation of energy, the power dissipated in the liquid is converted to heat within the liquid, with the liquid being heated at a rate proportional to the amount of power being dissipated.
Power, "P", is an exact function of torque "T" and rotative speed "N", and is calculated from the relation EQU P=TN/C ,
where "C" is a constant dependant upon the units of measurement used for P, T, and N.
Heretofore, the heated liquid, usually water, has been discharged as hot water while being simultaneously replaced by cold water. This is referred to as "sensible heating"; i.e., the effect can be physically sensed as a temperature increase. For fresh water density of 8.33 lb./gal. and specific heat capacity of 1 BTU/lb./F.degree., power "P" in horsepower (hp), and entering cooling water temperature "T.sub.in " and leaving temperature "T.sub.out " (both in .degree.F.), prior art cooling water flow, or consumption, rate "Q.sub.s " is, in U.S. GPM (gallons per minute): EQU Q.sub.s (P)[(42.4 BTU/min.)/hp]{1/[1 BTU/(lb.F.degree.)]}* (1 gal./8.33 lb.)[1/(T.sub.out -T.sub.in)],
which reduces to EQU Q.sub.s =5.1 * P/(T.sub.out -T.sub.in)
T.sub.out is typically 160.degree. F. and approximately 212.degree. F. maximum, while T.sub.in is typically 55.degree. F. and 32.degree. F. minimum. The 32.degree. F. and 212.degree. F. are academic extremes and are rarely if ever encountered, as cooling water that cold in the quantities required is not economically available while a leaving temperature of 212.degree. F. places a prior art water brake in danger of imminent explosion.
Substituting in and solving the above, Q.sub.s is typically 0.05, and academically at least 0.03, U.S. GPM per hp, which yields typical and minimum consumption rates of 20 GPM and 12 GPM respectively for, as an example, a 400 hp power source.
Prior art water brakes have been incapable of sustained biphase operation (i.e., with liquid and vapor in equilibrium) due to inability to separate steam from water and allow the steam to leave in quantities sufficient to avert excessive internal pressure from arising, whereas the present invention instead retains the heated water until it vaporizes and is discharged as steam. In so doing, the rate of cooling water consumption for sustained operation is greatly reduced; for the same operating conditions cited above and utilizing the 970 BTU/lb. latent heat of vaporization of fresh water, the present invention's consumption rate "IQ.sub.v ", again in U.S. GPM, is EQU Q.sub.v =(P)[142.4 BTU/min.)/hp]{1 lb./[970 BTU+(212-T.sub.in)BTU/F.degree.]}(1 gal./8.33 lb.),
which reduces to EQU Q.sub.v 5.1{P/[970+(212-T.sub.in)]}GPM
for cooling water either at or below boiling point, which further reduces to EQU Q.sub.v 0.005*P GPM
for cooling water at its boiling point.
Substitution and solving yields present invention cooling water consumption rates of 0.0045 GPM per hp for 55.degree. F. cooling water, and 0.005 GPM per hp for 212.degree. F. cooling water; only 2 GPM for the 400 hp example above, or an order of magnitude (90%) less than the prior art, sensible heat water brake.
Additionally, the present invention can readily utilize cooling water entering at any temperature up to the boiling point while prior art waterbrakes typically require a large temperature difference between cold water entering and hot water leaving, and must be derated in capacity if this temperature difference is not available.
To accommodate differing and varying torque inputs in most applications, water brake retarders and, especially, dynamometers must have an adjustment or control capability; i.e., the amount of torque load they present their power sources must be controllable. In water brakes, this is most commonly done by adjusting the amount of water contained within the apparatus's housing and therefore in motion between rotor and stator. Operation must, in some applications, be controllably rangeable over a ratio of 50 to 1 in torque load.
The present invention belongs also to another class of invention, the mechanical liquid vaporizer. There are other mechanical liquid vaporizers in the prior art, but none are known to the applicant to be useable in sustained commercial operation as a retarder or dynamometer without disadvantage or deficiency as described immediately below and, in many cases, without addition of a heat transfer fluid loop. Only mechanical liquid vaporizers driven by moving shafts, whether rotatitive or linear, are considered herein; those utilizing electrical discharges, refrigerants, gasses, and the like are outside the scope of the present invention.
U.S. Pat. No. 3,198,191 (Wyszomirski, 1965), represented, specified, and claimed only as a heat generator, was, like the present invention, also a tangential water brake. Its squirrel cage impeller blades, however, by their very nature being not of full radial depth, precluded large adjustability of amount of water being impacted by the vanes or blades and therefore could not provide sufficient rangeability of torque load for general use as a retarder or dynamometer. Additionally, the squirrel cage blades and unobstructed inner chamber within the inner diameter of those blades allowed steam egress but did nothing to separate, remove, and recirculate particles of water entrained in the rapidly flowing steam, resulting in undesireably wetter steam at low operating speeds. Moreover, the construction of this device necessitated use of a relatively complex steam discharge tube assembly comprising inner and outer tubes, bearing, seal, leakage drain hole, and closely machined fits.
U.S. Pat. No. 1,149,938 (Naglevoort, 1915) was a tangential water brake but did not vaporize its cooling water.
U.S. Pat. No. 14,277,020 (Grenier, 1979) was a water brake of the viscous shear type and was intended primarily to heat liquid in a two-loop heat exchange system, but could also vaporize the liquid within the water brake. It was not, however, adjustable in torque load, and lacked means of discharging safely large quantities of steam and of separating entrained water particles from the steam.
U.S. Pat. No. 2,344,075 (Beldimano, 1944) was also a water brake and also used a heat transfer loop, in this case a water jacket surrounding the oil-filled water" brake. No steam generation or controllability were referred to.
U.S. Pat. No. 3,791,349 (Schaefer, 1974) was a steam generator, but not adjustable in torque load. Additionally, it utilized rapidly repeated water hammer, or fluid shock, within closed tortuous passageways; such phenomena are not normally regarded as conducive to long and troublefree life.
U.S. Pat. No. 4,781,151 (Wolpert et al., 1988) and U.S. Pat. No. 4,271,790 (Ahmed et al., 1981) were other embodiments of water brakes, neither adjustable in torque load and both utilizing two-loop heat transfer circuits.
U.S. Pat. No. 4,115,027 (Thomas, 1978) and U.S. Pat. No. 5,003,829 (DeConti and Quenneville, 1991) are representative of water cooled mechanical friction brakes (typically of drum, disk, or band styles), which can generate steam and are responsive to control actions over large ranges of operation. Under sustained heavy load, however, they require frequent replacement of sacrificial wear pads or blocks, and are prone to rotor failure caused by heat distortion and material heat checking. Depending upon aggressiveness of friction material selected, which must be carefully balanced with other factors including wear rate of friction pad material, they can also chatter or become unstable in torque load, particularly at very light or heavy load levels.
The above are representative of prior art waterbrakes which do not vaporize their cooling water (or other fluid) and hence require relatively large coolant flow rates, and of mechanical liquid vaporizers which lack the appropriate attributes of a water brake to an extent sufficient to preclude extended or sustained use as retarders or dynamometers.
The present invention is therefore a novel and very useful solution to prior art limitations in that it is a water brake which requires little cooling water and which discharges steam, instead of hot water, as either end product or byproduct, as the user wishes, and therefore can be used as steam generator, motion retarder, or dynamometer, either individually or simultaneously in combination. It generates steam with an efficiency of very nearly 100% since, except for minor radiation and convection losses from the apparatus's housing (which can themselves be made even smaller by covering the housing with thermal insulation), power entering through the shaft is all converted to thermal energy in the heating and vaporizing of water. Testing has shown it to be stable in speed and in torque load imposed upon a prime mover. Further, it's of simple design, easily manufactured and suitable for robust construction, and operates equally well in any angle of orientation, i.e., with shaft horizontal, vertical,,or any angle between.